A  Comparative  Study  of  the  Chemical 
Behavior  of 

Pyrite  and  Marcasite. 


THESIS  PRESENTED  TO  THE  FACULTY  OF  THE  DEPARTMENT 
OF  PHILOSOPHY  OF  THE  UNIVERSITY  OF  PENNSYL¬ 
VANIA  FOR  THE  DEGREE  OF  DOCTOR 
OF  PHILOSOPHY. 


'  By  Amos  Peaslee  Brown. 


■* 

Read,  before  the  American  Philosophical  Society, 

May  18,  18^4. 


/)  Reprinted  June  19,  1894,  from  Proc.  Amer.  Philos,  Soc.,  Vol.  xxxiii, 


\ 


/  * 


3 


A  Comparative  Study  of  the  Chemical  Behavior  of  Pyrite  and  Marcasite. 


By  Amos  Peaslee  Brown. 

( Bead  before  the  American  Philosophical  Society,  May  18,  189 4. ) 

"While  much  has  been  done  in  the  way  of  investigating  the  chemical 
properties  and  constitution  of  the  various  artificial  chemical  compounds, 
comparatively  little  attention  has  been  paid  to  the  constitution  of  the 
naturally  occurring  chemical  compounds.  The  carbon  compounds,  for 
instance,  have  in  an  immense  number  of  cases  been  investigated  with 
sufficient  exactness  to  allow  of  our  expressing  their  constitution  by  means 
of  structural  formulte,  but  to  how  many  minerals,  aside  from  the  simplest 
compounds,  can  we  assign  structural  formulae  that  are  based  on  any 
knowledge  that  we  possess  of  their  reactions?  It  is  true  that  much  has 
been  done  in  the  way  of  the  artificial  production  of  minerals,  and  some 
knowledge  of  the  constitution  of  certain  minerals  has  been  arrived  at  by 
a  study  of  their  decomposition  products,  but  very  little  in  comparison  to 
the  extent  of  the  field.  There  are  probably  several  reasons  for  this  neglect 
of  the  study  of  the  chemical  properties  and  constitution  of  minerals,  as 
want  of  homogeneity  in  the  minerals  themselves,  difficulty  of  procuring 
or  separating  pure  material  for  investigation,  and  similar  difficulties 
which  do  not  so  frequently  occur  with  artificial  compounds.  It  thus  hap¬ 
pens  that  mineral  chemistry  is  not  as  much  studied  as  it  deserves  to  be. 
Certain  groups  of  minerals  have,  however,  received  some  attention  ;  for 
instance,  Prof.  F.  W.  Clarke  has  been  carrying  on  a  very  interesting  series 
of  investigations  on  the  constitution  of  certain  silicates  which  have  been 
productive  of  most  valuable  results.  The  natural  sulphides,  sulpliarsen- 
ides  and  sulpho-salts  present  some  very  interesting  problems  in  regard  to 
their  constitution,  and  it  was  with  a  view  of  adding  to  out  knowledge  of 
the  chemical  behavior  of  two  of  these  sulphides  that  I  undertook  the 
series  of  investigations  about  to  be  described. 

The  compound  FeS2  is  found  in  nature  in  two  well-known  forms— the 
one  Pyrite  (the  isometric  mineral)  and  the  other  Marcasite  (the  ortho¬ 
rhombic  mineral).  Since  the  separation  of  the  two  names  from  the  gen¬ 
eral  term,  pyrites,  it  has  been  recognized  that  the  orthorhombic  form  is 
lighter  in  color  and  also  of  lower  specific  gravity  than  the  isometric  form. 
From  early  times,  also,  the  greater  tendency  of  “white  pyrites,”  or  mar¬ 
casite,  to  decompose  in  the  air  was  well  known. 

Pyrite,  the  form  which  resists  atmospheric  weathering  most  thoroughly, 
is  of  a  bright  brass-yellow  color  and  metallic  lustre,  breaking  with  an 
uneven  conchoidal  fracture,  but  bright  on  the  surface  of  fracture.  It 
crystallizes  in  the  isometric  system  in  forms  showing  generally  pentagonal 
hemihedrism.  Its  specific  gravity  ranges  from  4.8  to  5.2,  averaging  some 
what  over  5.  The  brass-yellow  crystals  are  generally  quite  unaltered  in 
the  air. 

REPRINTED  JUNE  19,  1894,  FROM  rROC.  AMER.  PHILOS.  SOC.,  VOL.  XXXIII. 


2 


Marcasite,  on  the  other  hand,  has  a  pale  greenish  or  grayish  yellow 
color,  an  uneven  lracture,  which  shows  a  somewhat  fibrous  structure, 
and  generally  but  little  lustre  on  the  surface  of  fracture.  It  crystallizes 
in  the  orthorhombic  system,  very  commonly  in  twins  or  radiated  fibrous 
masses.  It  is  not  very  permanent  in  moist  air,  but  readily  decomposes 
and  largely  into  FeS04.  The  chemical  formula  of  either  form,  calculated 
from  quantitative  analyses,  is  the  same,  FeS2  or  Fe  =  46. 67  % ,  S  =  53. 33  % . 

The  chemical  study  of  these  two  minerals  has  been  mainly  confined  to 
the  formation  of  one  of  them  artificially  and  to  a  few  experiments  on 
their  relative  decomposability.  Pyrite  has  been  made  artificially  in  a 
number  of  ways  ;  marcasite  has  not  as  yet  been  formed  artificially.* * * §  In 
1836,  Wohler f  produced  cubes  and  octahedra  of  pyrite  by  subjecting  a 
mixture  of  ferric  oxide,  flowers  of  sulphur  and  ammonium  chloride  to  a 
temperature  a  little  above  the  volatilizing  point  of  the  ammonium  chlo¬ 
ride.  The  resulting  mass  was  washed  to  isolate  the  crystals  from  the 
accompanying  pulverulent  matter.  Stanislas  Meunier  \  modified  this 
method  by  mixing  equal  parts  of  ferrous  carbonate  and  sulphur  and  heat¬ 
ing  in  glass  tubes  over  a  moderate  flame.  When  the  excess  of  sulphur 
has  been  driven  off,  there  remains  a  black  powder  containing  a  consider¬ 
able  percentage  of  cubes  of  pyrite.  Dana  §  states  that  pyrite  may  be 
made  “  by  slow  reduction  of  ferrous  sulphate  in  presence  of  some  carbon¬ 
ate.”  Baubigny  ||  produced  FeS2  as  a  crystalline  crust  by  acting  on 
metallic  iron  by  a  solution  of  S02  in  water  (H2S03)  in  closed  tubes  and 
at  a  temperature  of  200°.  As  neither  this  experiment  nor  the  one  imme¬ 
diately  preceding  it  shows  that  the  crystals  were  isometric,  it  is  possible 
that  both  of  them  may  be  marcasite.  Henri  Saint  Claire  Devilled  pro¬ 
duced  cubes  of  pyrite  by  melting  a  mixture  of  potassium  sulphide  (K2S) 
and  iron  sulphide  (FeS)  in  presence  of  excess  of  sulphur.  This  reaction, 
if  correct  as  to  the  cubical  product,  is  a  remarkable  one,  as  I  should  rather 
expect  marcasite  to  result  under  such  conditions.  Senarmont  **  produced 
FeS2  by  decomposing  a  salt  of  iron  by  an  alkaline  sulphide  at  an  elevated 
temperature  in  sealed  glass  tubes.  The  product  was  an  amorphous  black 
powder,  not  altering  on  exposure  to  air  and  not  attacked  by  hydrochloric 
acid.  This  may  have  been  pyrite,  as  marcasite  is  readily  decomposed  by 
moist  air.  Raminelsberg,tf  in  1862,  made  FeS2  pseudomorphs  after  ferric 
oxide  (Fe203)  by  passing  a  current  of  hydrogen  sulphide  over  it  at  a  tem¬ 
perature  between  100°  and  a  red  heat.  The  product  of  this  reaction  would 
likely  be  pyrite. 

In  nature  it  would  seem  that  in  most  cases  the  sulphide  of  iron  first 

*Doelter,  Zeit.fiir  Kryst.,  xi,  31,  1885 ;  cf.  Dana,  Syst.  Min.,  1892. 

f  Pogg.  Ann.,  xxxvii,  p.  238. 

X  Les  Methodes  de  Synthase  en  Mineralogie,  S.  Meunier,  1891. 

§  J.  D.  Dana,  System  of  Mineralogy ,  edition  of  1868,  p.  64. 

H  S.  Meunier,  Synth.  Min.,  p.  279. 

If  Cited  in  Diet.  Chem.  of  Wartz,  by  E.  Wilm,  article  “  Iron,”  T.  i,  p.  1414. 

**S.  Meunier,  Synth.  Min.,  p.  285. 

ft  Jour,  fiir  Praktische  Cliemie,  T.  lxxxviii,  p.  266. 


3 


formed  is  FeS,  but  probably  by  action  of  a  ferric  salt,  or  carbonic  acid 
(H.,C03),  causing  a  loss  in  iron  FeS2  results,  and  this  is  almost  always 
pyrite.  On  the  other  hand,  where  ferrous  sulphate  has  been  reduced  by 
slow  action  of  decomposing  organic  matter,  the  resulting  sulphide  seems 
to  be  generally  marcasite,  which  if  not  in  crystals  may  be  recognized  by 
its  ready  weathering  to  ferrous  sulphate  (FeS04).  This  compound  may, 
however,  in  many  cases,  be  a  mixture  of  pyrite  and  marcasite,  as  much 
pyrite  seems  to  be.*  These  several  ways  in  which  pyrite  may  be  formed 
are  of  importance  as  indicating  the  condition  of  the  iron  in  the  compound, 
and  will  be  referred  to  later  on. 

Equally  important  as  bearing  on  the  constitution  of  these  sulphides  are 
the  observations  that  have  been  made  as  to  their  decomposition  under  at¬ 
mospheric  agencies.  On  exposing  crystallized  pyrite  to  atmospheric 
weathering  it  is  generally  found  that  but  little,  if  any,  change  takes  place 
even  in  a  year’s  time,  while  crystallized  marcasite,  under  the  same  condi¬ 
tions,  shows  a  rapid  weathering.  The  main  product  of  the  weathering 
of  pyrite  in  nature  is  the  compound  limonite,  Fe403(0H)6,  which  occurs 
in  large  quantities  in  nature,  evidently  derived  from  pyrite.  Its  pseudo- 
morphs  after  pyrite  are  very  common.  On  the  other  hand,  when  marca¬ 
site  weathers  a  very  large  percentage  of  ferrous  sulphate  (FeS04)  is 
formed  with  a  comparatively  small  percentage  of  limonite,  unless  the 
marcasite  decomposes  underground  and  under  pressure,  when  limonite  is 
largely  produced. f  Much  of  the  excess  of  sulphur  with  marcasite  is 
changed  to  sulphuric  acid,  but  with  pyrite  much  remains  behind  as  sul¬ 
phur.  Some  comparison  of  the  rate  of  oxidation  of  these  two  minerals 
in  the  air  is  afforded  by  an  examination  of  specimens  in  a  collection. 
Here  it  will  be  found  that  most  of  the  pyrite  is  unchanged,  but  nearly 
every  specimen  of  marcasite  will  show  tarnish  if  no  other  sign  of  oxida¬ 
tion,  and  often  a  considerable  coating  of  oxide  can  be  seen,  or  a  white 
efflorescence  of  FeS04. 

Chemical  investigations  of  these  two  minerals  have  been  mainly  in  the 
way  of  analysis,  but  some  experiments  have  been  made  in  the  way  of 
studying  their  relative  behavior  towards  certain  reagents.  Before  de¬ 
scribing  my  experiments,  it  will  be  necessary  to  briefly  mention  some  of 
these. 

Both  minerals  are  decomposed  by  a  solution  of  silver  nitrate  and  gold 
chloride,  the  decomposition  taking  place  quite  rapidly.:}:  My  experiments 
in  this  connection  are  mentioned  later. 

A.  A.  Julien  \  has  shown  that  different  samples  of  pyrite  show  a  differ¬ 
ence  in  their  reaction  with  bromine  vapor.  His  experiments  consisted  in 
exposing  finely  ground  pyrite  to  the  action  of  bromine  vapor  at  the  tem- 

*  Compare  A.  A.  Julien,  “Decomposition  of  Pyrite,”  Ann.  N.  Y.  Acad.  Sci.,  Yols.  iii 

and  iv. 

t  Blum,  Pseudomorphosen,  1843,  pp.  197-199. 

JS.  Meunier,  Synth.  Min.,  p.  309. 

I  A.  A.  Julien,  Ann.  N.  Y.  Acad.  Sci.,  Vol.  iv,  pp.  151, 155. 


4 


perature  of  the  air  for  twelve  hours.  The  residue  was  extracted  with 
dilute  H?S04,  which  removed  the  iron  rendered  soluble  (bromide),  and 
the  iron  was  then  determined  in  this  solution.  The  percentage  of  iron 
that  had  dissolved  varied  from  2.43  to  15.20  per  cent.,  although  all  sam¬ 
ples  tested  are  described  as  pyrite.  He  also  tried  the  action  of  bromine 
in  aqueous  solution,  but  the  reaction  was  too  rapid  to  give  any  compara¬ 
tive  results. 

Much  more  important  are  the  results  obtained  in  tbe  oxidation  of  these 
minerals  by  the  electric  current  as  conducted  by  Prof.  Edgar  F.  Smith,* 
and  it  was  the  remarkable  results  that  were  then  obtained  that  induced 
me  to  continue  the  study  of  the  comparative  reactions  of  these  two  min¬ 
erals  Smith  found  that  a  current  which  would  completely  oxidize  the 
sulphur  in  marcasite  in  a  given  time  would  oxidize  less  than  half  of  the 
sulphur  in  pyrite  in  the  same  time.  This  remaining  sulphur  was  held 
very  tenaciously,  though  the  mineral  was  subjected  to  more  powerful  cur¬ 
rents  and  longer  continued  action  than  in  the  case  of  marcasite  or  pyrrho- 
tite.  Finally,  by  adding  an  equal  quantity  of  CuO,  and  using  a  more 
powerful  current,  all  ot  the  contained  sulphur  was  oxidized.  Previous 
to  the  addition  of  CuO  but  21  or  22  per  cent  of  the  sulphur  was  oxidized. 
In  concluding  the  article  above  referred  to  the  author  questions  whether 
the  crystalline  form  alone  can  make  this  difference  in  the  action  of  the 
two  minerals  when  under  the  influence  of  the  current. 

The  two  samples  of  pyrite  and  marcasite  that  I  selected  for  the  follow¬ 
ing  study  were  chosen  after  considerable  examination  of  material  as  being 
typical  of  the  two  forms  of  FeS2.  The  pyrite  was  from  the  hematite 
mines  of  Elba.  It  is  exceptionally  pure  and  free  from  decomposition  or 
tarnish.  Before  deciding  on  it  finally  pieces  were  ground  and  polished 
and  then  examined  under  the  microscope  with  powers  ranging  from  50  to 
200  diameters,  in  order  to  see  if  it  contained  any  enclosures  or  varied  in 
texture.  It  was  perfectly  homogeneous  and  showed  no  enclosures.  It 
took  a  high  polish.  The  crystals  showed  the  combination  of  octahedron 
and  pentagonal  dodecahedron  O  -{-  £§-.  Some  of  the  crystals  were  coated 
in  places  with  scales  of  hematite,  but  this  was  all  carefully  removed  in 
breaking  up  material  for  experiment.  The  color  was  bright  brass-yellow, 
the  specific  gravity  was  determined  as  5.179.  The  marcasite  was  from  the 
zinc  mines  of  the  Subcarboniferous  of  Joplin,  Jasper  county,  Mo.,  finely 
crystallized  in  polysynthetic  twinnings.  The  freshly  broken  crystals  show 
a  greenish  yellow  color,  almost  white,  but  they  tarnish  readily  with  bluish 
or  brownish  colors.  No  gangue  was  present,  everything  dissolving  com¬ 
pletely  in  nitric  acid.  This  marcasite  was  examined  with  the  microscope  in 
the  same  way  as  the  pyrite  ;  it  did  not  take  such  a  high  polish  on  account  of 
a  fibrous  structure,  but  no  foreign  matter  was  found  with  a  power  of  200 
diameters.  Its  color  was  uniform  throughout,  showing  that  no  pyrite 
was  present.  The  specific  gravity  as  determined  was  4.844. 

In  preparing  material  for  experiment  only  sufficient  was  ground  forirn- 

*Jour.  Franklin  Inst.,  Vol.  cxxx,  pp.  152-154. 


5 


mediate  use  to  avoid  any  cliance  of  oxidation  of  the  ground  material ; 
the  stock  samples  of  the  two  minerals  broken  to  nut  size  were  kept  in 
stoppered  bottles.  The  grinding  of  material  was  continued  as  long  as 
grit  appeared,  but  no  bolting  was  resorted  to. 

As  the  experiments  of  Prof.  Smith  on  oxidation  by  the  electric  current 
showed  such  remarkable  results,  my  first  experiments  were  on  oxidation. 
As  an  oxidizing  agent  potassium  permanganate  (KMn04)  was  used,  sev¬ 
eral  strengths  of  which  were  tried  for  varying  intervals  of  time  with  each 
mineral,  and  the  amount  of  sulphur  oxidized  to  sulphuric  acid  determined 
in  the  liquid  by  precipitating  as  barium  sulphate.  The  object  was  to 
secure  a  complete  series  of  results  which  would  show  the  comparative 
rates  of  oxidation  of  the  sulphur  in  the  two  minerals.  Neutral  aqueous 
solutions  of  the  potassium  permanganate  were  used,  and  the  strengths 
of  solution  employed  were  normal,  one  per  cent.,  three  per  cent,  and 
five  per  cent.;  the  periods  of  oxidation  extending  over  one,  two,  three, 
four  and  five  hours,  and  the  entire  series  being  performed  at  ordinary 
temperatures  and  at  100°.  As  duplicate  determinations  were  made  in  the 
majority  of  cases  (I  made  about  130  determinations  of  su  pliur  as  barium 
sulphate),  this  work  consumed  a  large  amount  of  time  and  prevented  as 
fall  a  study  of  some  other  reactions  of  the  two  minerals  as  had  been  orig¬ 
inally  intended.  The  following  are  the  detailed  descriptions  of  my  pro¬ 
cesses  and  results  : 

Action  of  Normal  Potassium  Permanganate  Solution  at 
Ordinary  Temperature. 

These  oxidations  were  performed  as  follows  :  Two-tentlis  of  a  gram 
of  the  finely  powdered  mineral  was  placed  in  a  stoppered  bottle  of  about 
100  c.c.  capacity,  then  50  c.c.  of  the  permanganate  solution  added  and 
the  contents  of  the  bottle  violently  shaken  to  break  up  lumps.  This 
shaking  was  repeated  about  every  fifteen  minutes  while  the  oxidation 
lasted.  The  temperature  of  the  room  was  at  the  same  time  recorded.  As 
stated,  the  oxidation  was  continued  for  one,  two,  three,  four  and  five 
hours  with  each  mineral,  making  at  least  ten  experiments  necessary  for 
each  strength  of  solution.  After  the  solution  had  acted  for  the  required 
time  it  was  rapidly  filtered  through  asbestos  with  aid  of  the  filter  pump, 
the  filtrate  transferred  to  a  beaker,  20  c.c.  of  concentrated  hydrochloric 
acid  added,  and  the  whole  heated  until  all  manganese  was  reduced  to 
manganous  chloride.  If  not  too  acid  the  solution  was  then  diluted  to 
about  300  c.c.  and  the  sulphuric  acid  precipitated  as  barium  sulphate. 
When  very  acid,  excess  of  hydrochloric  acid  was  removed  by  evaporation 
or  by  adding  ammonia,  the  ammonium  chloride  seeming  to  facilitate  the 
precipitation.  The  precipitate  was  washed  with  hot  water  and  then 
weighed.  All  precipitations  were  made  at  boiling  temperature  and  di¬ 
gested  hot  for  at  least  two  hours,  and  then  cold  for  at  least  twelve  hours 
more  before  filtering.  The  filtrates  from  most  of  the  cold  tests  were  re- 


6 


duced  with  metallic  zinc  and  titrated  with  permanganate,  but  no  iron  was 
found  in  the  solution. 

The  two  minerals  did  not  present  the  same  appearance  when  acted  on 
by  the  oxidant.  Pyrite  retained  its  color  and  seemed  as  pulverulent  as 
when  the  permanganate  was  added,  but  marcasite  immediately  on  the 
addition  of  the  reagent  became  coated  with  manganese  dioxide,  took  on  a 
brownish  color,  and  showed  a  tendency  to  cake  together  and  stick  to  the 
sides  of  the  bottle,  so  that  it  was  with  difficulty  dislodged.  This  tendency 
of  the  marcasite  was  more  marked  with  stronger  solutions  of  the  perman¬ 
ganate  and  was  doubtless  the  cause  of  much  of  the  irregularity  that  will 
be  noticed  in  the  results.  The  reason  for  this  difference  in  action  of  the 
reagent  on  the  two  minerals  will  be  discussed  later  on. 

The  percentages  of  sulphur  oxidized  in  the  two  minerals  by  this  method 
are  shown  in  the  following  table,  where  all  results  that  were  obtained  are 
recorded.  The  figures  show  the  percentages  of  sulphur  oxidized,  calcu¬ 
lated  on  the  basis  of  FeS2  equal  to  one  hundred  per  cent.  It  will  be  noted 
that  the  four-hour  oxidation  of  marcasite  shows  a  result  that  is  less  than 
the  two-hour.  This  was  due  to  caking  of  the  mineral  against  the  w  alls  of 
the  bottle,  which  prevented  much  of  it  from  coming  in  contact  with  the 
solution.  On  the  whole,  this  series  was  about  the  most  satisfactory  of  the 
cold  experiments  with  KMn04,  the  action  of  this  dilute  solution  being 
less  rapid  and  hence  more  even  than  that  of  the  more  concentrated  solu¬ 
tions  ;  naturally  the  action  ceases  with  a  certain  dilution,  and  hence  the 
four-  and  five-hour  oxidations  of  pyrite  are  about  equal. 


Table  Showing  the  Relative  Oxidation  of  Sulphur  in  Pyrite  and  Marcasite 
by  a  N.  Solution  of  KMn  0i  at  22°. 


Mineral. 

1-Hour. 

2-Hour. 

3-Hour. 

4-Hour. 

5- Hour. 

Pyrite  with  N.  solution 

KMn04 . . 

.78 

1.17 

1.88 

1.74 

1.72 

Marcasite  with  N.  solution 

KMn04 . 

1 

1.07 

1.86 

2.04 

(1.25) 

2.38 

The  curves  formed  by  plotting  these  results  on  rectangular  coordinates 
are  shown  in  Pis.  i  and  ii.  They  are  marked  22°  Mx  for  the  marcasite 
and  22°  Px  for  the  pyrite. 


Action  of  1  Per  Cent.  Potassium  Permanganate  Solution  at 
Ordinary  Temperature. 

This  and  also  the  two  following  series  were  performed  as  described 
under  normal  solution  above.  At  least  two  experiments  were 
tried  with  each  mineral  in  this  and  the  two  following  cold  oxidations, 
and  whenever  a  result  was  notably  higher  or  lower  than  its  dupli- 


cate  a  third  or  fourth  was  tiied.  The  tendency  of  the  maroasite  to  cake, 
noted  in  the  previous  series,  became  still  more  marked  here,  and  is  doubt¬ 
less  the  cause  of  one  of  the  four-hour  oxidations  (marked  by  parenthesis) 
being  notably  lower  than  the  three-hour.  Such  a  result  is  obviously 
incorrect.  On  the  other  hand,  the  result  in  the  tliree-hour  column  which 
is  placed  in  parenthesis  is  the  highest  obtained.  This  experiment  was 
made  at  the  same  time  as  the  one  showing  1.93  per  cent.,  but  the  room 
was  very  warm  (25°),  which  may  in  part  account  for  this  high  result.  It 
will  be  noticed  that  the  oxidation  of  the  pyrite  seems  to  stop  at  the  three- 
hour  trial,  those  following  showing  no  appreciable  increase.  This  is  well 
seen  in  the  graphic  representation  of  these  oxidations  (Pis.  i  and  ii). 


Table  Showing  the  Relative  Oxidation  of  Sulphur  in  Pyrite  and  Marcante 
by  a  1  Per  Cent.  Solution  of  KMnO 4  at  22°. 


Mineral. 

1-Hour. 

2-Hour. 

3-Hour. 

4-Hour. 

5-Hour. 

Pyrite  with  1  per  cent,  solution 
KMn04  cold. 

1.72 

1.71 

1.38 

1.47 

1.85 

1.87 

1.79 

1.90 

1.70 

1.89 

Marcasite  with  1  per  cent,  solu¬ 
tion  KMn04  cold. 

1.16 

1.28 

1.29 

1.13 

1.93 

2.19 

(3.92) 

1.95 

(1.56) 

2.69 

2.01 

2.15 

2.55 

Action  op  3  Per  Cent.  Solution  of  Potassium  Permanganate  at 
Ordinary  Temperature. 

The  conditions  of  this  series  of  experiments  were  the  same  as  those 
of  llie  last.  The  tendency  of  the  results  to  fluctuate  instead  of  show¬ 
ing  a  gradual  progression  is  now  very  marked.  One  of  the  one-liour 
pyrite  oxidations  shows  more  sulphur  oxidized  than  is  shown  by  any 
other  individual  result  of  the  series.  No  explanation  can  be  offered  for 
such  a  discrepancy  as  this.  On  the  other  hand,  the  high  result  shown  in 
the  three-hour  oxidation  is  quite  easily  explained  by  the  marcasite  hav¬ 
ing  been  little,  if  any,  caked  in  this  experiment.  The  two  low  results  of 
pyrite  three  hour  and  marcasite  four-hour  oxidations  are  readily  explica¬ 
ble  on  the  ground  of  caking  of  the  material.  As  the  barium  sulphate 
was  often  determined  several  days  after  the  oxidation  was  completed,  it 
is  obvious  that  no  reliable  notes  could  be  made  concerning  the  caking  or 
non  caking  of  the  mineral  in  the  permanganate.  With  this  strength  of 
solution  it  is  evident,  too,  that  the  main  action  of  the  permanganate  is 
complete  at  the  end  of  one  hour,  notably  in  the  case  of  the  marcasite,  and 
it  is  only  when  very  vigorous  agitation  exposes  fresh  surfaces  of  the  min¬ 
eral  to  the  action  of  the  KMn04  that  any  further  action  can  take  place. 
We  therefore  see  that  marcasite  in  one  hour  gives  up  as  much  sulphur  as 
in  live  hours,  and  this  is  very  graphically  shown  on  PI.  i. 


8 


Table  Showing  the  Relative  Oxidation  of  Sulphur  in  Pyrite  and  Marcasite 
by  a  3  Per  Cent.  Solution  of  KMnOi  at  22°. 


Mineral. 

1-Hoor. 

2-Hour. 

3-Hour. 

4 -Hour. 

5-Hour. 

Pyrite  with  3  percent,  solution 
KMn04  cold. 

1.65 

(3.55) 

2.23 

2.31 

2.80 

(1.58) 

2.47 

2.62 

2.81 

2.28 

Marcasite  with  3  per  cent,  solu¬ 
tion  KMn04  cold. 

2.72 

2.87 

2.17 

2.33 

2.87 

(3.31) 

2.88 

(1.89) 

2.83 

2.77 

Action  of  5  Per  Cent.  Solution  of  Potassium  Permanganate  at 
Ordinary  Temperature. 

In  this  series,  as  in  the  last,  the  action,  as  far  as  pyrite  was  concerned, 
was  practically  complete  at  the  expiration  of  the  first  hour,  but  in  the 
case  of  the  marcasite  this  point  was  not  reached  until  probably  the  end  of 
the  second  hour,  and,  in  fact,  in  one  case  was  progressive  to  the  end. 
But  one  very  great  discrepancy  is  to  be  noted  here  in  the  three-hour  col¬ 
umn  with  marcasite.  The  low  result  in  the  next  column  is  explained 
by  caking. 


Table  Showing  the  Relative  Oxidation  of  Sulphur  in  Pyrite  and  Marcasite 
by  a  5  Per  Cent.  Solution  of  KMn  Oi  at  22°. 


Mineral. 

1-Hour. 

•  2-Hour. 

3-Hour. 

4-Hour. 

5-Hour. 

Pyrite  with  5  per  cent,  solution 
KMn04  cold. 

2.39 

3.15 

3.03 

3.15 

3.22 

2.32 

2.89 

2.79 

3.24 

Marcasite  with  5  per  cent,  solu¬ 
tion  KMn04  cold. 

2.10 

2.52 

3.06 

3.76 

3.82 

(5.83) 

2.77 

3.16 

(2.44) 

3.39 

4.17 

This  series  finishes  the  experiments  at  ordinary  temperatures.  In  all  of 
them  the  action  was  comparatively  slight,  not  exceeding  at  most  10  per  cent, 
of  the  contained  sulphur  in  the  mineral  which  would  not  be  sufficient  to 
show  any  marked  difference  between  the  two  minerals  as  bearing  on  their 
constitution,  if  the  constitution  which  seems  to  be  indicated  by  subse¬ 
quent  experiments  (to  be  presently  described)  is  the  true  one. 

The  oxidations  with  potassium  permanganate  at  a  temperature  of  100° 
were  conducted  by  suspending  the  vessel  containing  the  mineral  and  so¬ 
lution  in  boiling  water.  Both  stoppered  bottles  and  thin  glass  flasks 
closed  with  perforated  corks  were  used  for  this  series  of  experiments. 
The  water  was  kept  continually  boiling  and  the  bottles  or  flasks  were  im¬ 
mersed  deep  enough  to  cover  that  portion  of  them  containing  the  per¬ 
manganate.  Six  or  eignt  oxidations  were  made  at  one  operation.  The 


9 


permanganate  solution,  after  it  had  acted  the  required  time,  was  treated 
as  in  the  experiments  conducted  at  ordinary  temperatures  described  above. 
Much  more  active  oxidation  took  place  at  this  temperature  (100°),  but 
the  tendency  of  the  mineral  to  cake  together  was  much  more  marked, 
and  now  this  took  place  with  pyrite  as  well  as  with  marcasite.  Moreover, 
the  deposition  of  manganese  dioxide  in  every  case  was  now  very  great, 
causing  often  a  stoppage  of  the  oxidation  until  it  could  be  dislodged.  As 
these  oxidation  experiments  had  already  occupied  much  time  only  one 
trial  was  now  made  at  each  concentration  of  solution  for  each  hour  from 
one  to  five,  unless,  as  before,  marked  discrepancies  occurred,  when  two 
or  more  trials  were  made.  The  series  of  results  are  hence  not  so  regular 
as  they  would  have  been  had  more  trials  been  made,  these  irregularities 
arising  from  the  difficulties  that  have  been  mentioned,  as  well  as  from  the 
fact  that  the  dilute  solutions  soon  became  exhausted,  and  that  all  solutions 
suffered  some  evaporation,  but  some  more  than  others,  causing  irregular 
strength  with  the  same  solution.  Nevertheless,  the  results  agree  in  kind 
with  those  obtained  at  ordinary  temperature,  but  differ  widely  in  degree. 
Whereas  at  ordinary  temperature  the  greatest  amount  of  sulphur  oxidized 
in  marcasite  by  the  five-hour  trial  with  o  per  cent,  permanganate  was  4.17 
per  cent.,  at  100°  this  became  16.36  per  cent,  or  about  four  times  as  much. 

Action  of  Normal  Potassium  Permanganate  Solution  at  100°. 

The  results  given  in  the  following  table  show  perhaps  more  strongly  than 
either  of  the  other  series  of  experiments  at  100°  the  effect  of  the  different 
disturbing  causes  that  have  been  mentioned.  It  especially  shows  the 
effect  of  caking  of  the  pyrite,  which  now  came  in  as  an  important  dis¬ 
turbing  factor.  The  result  of  this  caking  is  shown  in  the  three-  and  four- 
hour  results  with  pyrite,  both  being  very  low.  Marcasite,  on  the  other 
hand,  invariably  caked  and  stuck  to  the  bottom  of  the  bottle,  but  as  this 
was  a  constant  source  of  error  in  this  case,  the  results  show  a  gradual  and 
fairly  even  increase.  Irregular  results  with  marcasite  were  now  largely 
conditioned  by  the  evaporation  of  the  solution  or  by  the  fact  of  whether 
the  mineral  was  evenly  caked  over  the  inner  surface  of  the  vessel  or  con¬ 
centrated  in  spots.  The  result  of  this  latter  way  of  caking  will  be  better 
seen  in  some  of  the  subsequent  series  of  experiments. 


Table  Slcowing  the  Relative  Oxidation  of  Sulphur  in  Pyrite  aud  Marcasite 
by  «  Normal  Solution  of  KMnOi  at  100°  C. 


Mineral. 

l-IIOUR. 

2-Hour. 

3-Hour. 

4-Hour. 

5-Hour. 

Pvrite  with  N.  KMn04  at 

*100°  C . 

4.05 

4.72 

3  36 

2.04 

5.64 

Marcasite  with  N.  KMn04 

at  100°  C . 

3.17 

3.84 

3.76 

5.63 

5.61 

10 


Action  of  1  Per  Cent.  Potassium  Permanganate  Solution  at  100°. 

The  results  of  the  oxidation  shown  in  this  series  are  chiefly  remarkable  as 
still  further  illustrating  the  action  of  the  solution  on  the  caked  material, 
as  shown  in  the  four-  and  five-hour  trials  with  pvrite  and  the  four-hour 
trial  with  marcasite.  This  latter  shows,  too,  the  effect  of  having*  the 
caked  mineral  massed  in  one  spot.  With  this  exception,  the  marcasite 
oxidations  are  progressive  and  fairly  uniform  (the  two  hour  trial  falls 
somewhat  below  what  it  doubtless  should  be),  but  the  pyrite  shows  a 
sharp  fall  in  the  four-  and  five-hour  trials.  The  cause  of  this  has  been 
indicated.  That  the  concentration  of  the  solution  by  evaporation  caused 
an  increase  in  the  action  seems  to  be  indicated  by  the  result  of  the  five- 
hour  marcasite  oxidation,  but  this  is  much  more  strongly  marked  in  the 
5  per  cent,  series  in  the  case  of  pyrite,  which  will  be  referred  to  later  on. 


Table  Shouting  the  Relative  Oxidation  of  Sulphur  in  Pyrite  and  Marcasite 
by  a  1  Per  Gent.  Solution  of  KMnO 4  at  100°. 


Mineral. 

1-Hour. 

2- Hour. 

3-Hour. 

4-HOUR. 

5-Hour. 

Pyrite  with  1  per  cent.  KMn04 
at  100°  C. 

6.03 

6.98 

8.38 

6.11 

6.88 

Marcasite  with  1  per  cent. 
KMn04  at  100°  C. 

6.43  | 

- 

i  5.61 
6.25 

8.56 

7.40 

9.10 

Action  of  3  Per  Cent.  Solution  of  Potassium  Permanganate 

at  100°  C. 

The  average  results  of  this  series  of  oxidations  were  very  good,  with  the 
exception  of  two  members  of  the  series — the  marcasite  five-hour  trial  and 
the  pyrite  three-hour.  This  latter  was  repeated,  but  with  a  similar  low 
result.  Leaving  these  two  out  of  account,  the  average  results  show  a 
very  fairly  even  rate  of  progression,  which  have  been  brought  out  in  the 
diagram  (PI.  i).  It  is  evident  from  an  inspection  of  the  following  table 
that  the  marcasite  oxidations  of  the  four-  and  five-hour  trials  were  arrested 
by  some  disturbing  influence. 


Table  Showing  the  Relative  Oxidation  of  Sulphur  in  Pyrite  and  Marcasite 
by  a  3  Per  Cent.  Solution  of  KMnO±  at  100°  G. 


Mineral. 

1-Hour. 

2- Hour. 

3-Hour. 

4-HOUR. 

5-Hour. 

Pyrite  with  3  per  cent.  KMn04 
at  100°  C. 

5.52 

(7.01) 

5.77 

7.89 

6.81 

5.16 

10.73 

11.08 

Marcasite  with  3  per  cent. 
KMn04  at  100°  C. 

5.25 

5.50 

(8.67) 

7.97 

9.42 

9.80 

7  55 

11 


Action  of  a  5  Per  Cent.  Potassium  Permanganate  Solution 

at  100°  C.  • 

This  series  was  decidedly  the  most  satisfactory  of  the  experiments  con¬ 
ducted  at  100°  and,  with  the  exception  of  the  two-liour  oxidations,  in  case 
of  each  mineral  shows  a  remarkably  regular  increase  in  the  oxidation  of 
the  sulphur.  It  illustrates,  too,  in  the  case  of  the  pyrite,  the  effect  of  the 
concentration  of  the  solution  due  to  evaporation.  This  causes  a  more 
rapid  rise  in  the  results  after  three  hours’  action,  and  notably  between 
four  and  five  hours.  This  rise  is  not  so  well  seen  from  the  table  as  from 
the  plot  of  the  results  given  in  PI.  i.  Here  this  rising  of  the  curve  after 
three  hours  is  very  marked  as  contrasted  with  the  curve  of  the  marcasite 
oxidations. 


Table  Showing  the  Relative  Oxidation  of  Sulphur  in  Pyrite  and  Marrcasite 
by  a  5  Per  Cent.  Solution  of  KMn  04  100°. 


Mineral. 

1-Hour. 

2-HOUR. 

3-Hour. 

4-Hour. 

| 

5-Hour. 

Pyrite  with  5  per  cent.  KMn04 
at  100°  C . 

7.95 

7.52 

9.85 

11.86 

14.98 

Marcasite  with  5  per  cent. 
KMn04  at  100°  C . 

8.38 

8.38 

13.27 

14.85 

16.36 

The  graphic  representation  of  the  rate  of  oxidation  of  sulphur  in  these 
two  minerals  is  plotted  from  the  following  tables,  the  first  of  which  shows 
the  average  amounts  of  oxidation  of  sulphur  compiled  from  the  several 
small  tables  already  given.  In  this  compilation  I  have  omitted  a  few  of 
the  results  given  in  parentheses  in  the  small  tables  as  they  are  evidently 
incorrect,  and  would  vitiate  the  averages.  On  the  other  hand,  I  have  sim¬ 
ply  retained  the  parentheses  of  the  small  tables  in  certain  cases  where  but 
one  result  was  obtained.  The  second  table,  also  compiled  from  the  sev¬ 
eral  small  tables,  shows  for  the  experiments  at  ordinary  temperature  the 
result  of  selecting,  wherever  possible,  such  individual  determinations  as 
show  a  progressive  increase  in  the  oxidation.  In  this  way  more  regular 
series  of  results  are  obtained.  Inasmuch  as  the  variations  are  so  marked, 
this  method  of  selecting  results  seems  justified,  at  least  for  obtaining  a  com¬ 
parison  of  the  relative  rates  of  oxidation  of  the  sulphur  in  the  two  min¬ 
erals.  The  results  of  Table  i  are  plotted  in  PI.  i,  and  those  of  Table  ii  in 
PI.  ii. 


12 


I.  Table  Showing  Average  Relative  Oxidation  of  Sulphur  in  Pyrite  and 
Marcasite  by  Solutions  of  KMn.O 4  at  22°  and  at  100°.  See  PI.  i. 


Mineral. 

1-Hour 

2-Hour 

3-Hour 

1 

4-Hour 

5-Hour 

On  Pl. 

Pyrite  T-J-„  N.  cold . 

.78 

1.17 

1.38 

1.74 

1.72 

220 

Pi 

Marcasite  N.  cold . 

1.07 

1.86 

2.04 

(1.25) 

2.38 

220 

M1 

Pyrite  1  per  cent,  cold . 

1.71 

(1.43) 

1.86 

1.85 

1.80 

22° 

P2 

Marcasite  1  percent,  cold.... 

1.22 

1.21 

2.06* 

2.32* 

2.24 

22° 

m2 

Pyrite  3  per  cent,  cold . 

2.55 

2.27 

2.80* 

2.55 

2.55 

22° 

P3 

Marcasite  3  per  cent,  cold _ 

2.80 

2.25 

2  87* 

2.88* 

2.80 

220 

m3 

Pyrite  5  percent,  cold . 

2.77 

3.09 

2.77 

2.89 

3.02 

22° 

P4 

Marcasite  5  per  cent,  cold _ 

2.31 

3.41 

3.30* 

3  16 

3.78 

22° 

m4 

Pyrite  N.  100° . 

4.05 

4.72 

3.36 

2.04 

5.64 

100° 

Pi 

Marcasite  N.  100° . 

3.17 

3.84 

3  76 

5.63 

5.61 

100° 

Mi 

Pyrite  1  per  cent.  100° . 

6.03 

6  98 

8.38 

6.11 

6.88 

100° 

P2 

Marcasite  1  per  cent.  100° . 

6.43 

5.93 

8.56 

7.40 

9.10 

100° 

m2 

Pyrite  .3  per  cent.  100° . 

6.26 

6.83 

6.81* 

10.73 

11.08 

100° 

P3 

Marcasite  3  per  cent.  100° - 

6.47 

7.97 

9.42 

9.80 

(7.55) 

100° 

m3 

Pyrite  5  per  cent.  100° . 

7.95 

7.52 

9.85 

11.86 

14.98 

100° 

P4 

Marcasite  5  per  cent.  100c. . . . 

8.38 

8.38 

13.27 

' 

14.85 

1 

16.36 

100° 

II.  Table  Showing  Selected  Results  of  the  Oxidation  of  Sulphur  in  Pyrite 
and  Marcasite  by  Solutions  of  KMnO 4  at  22°.  See  PI.  ii. 


Mineral. 

1-HOUR 

2-Hour 

3-Hour 

4-Hour 

5-Hour 

On  Pl. 

Pyrite  N.  cold . 

.78 

1.17 

1.38 

1.74 

(1.72) 

22° 

Pi 

Marcasite  N.  cold . 

1.07 

1.86 

2.04 

(1.25) 

2.38 

220 

Mi 

Pyrite  1  per  cent,  cold . 

(1.71) 

1.47 

1.85 

1.90 

1  89 

22° 

P2 

Marcasite  1  per  cent  cold . 

1.16 

1.29 

1.93 

1.95 

2.15 

220 

Mo 

Pyrite  3  per  cent,  cold . 

(1.65) 

2.31 

2.80 

(2.62) 

2.81 

22° 

P3 

Marcasite  3  percent,  cold . 

(2.72) 

2.33 

2.87 

2.88 

2.83 

22° 

m3 

Pyrite  5  per  cent,  cold . 

2.39 

3.03 

3.22 

(2.89) 

3.24 

22° 

^4 

Marcasite  5  percent,  cold . 

2.52 

3.06 

3.82 

(3.16) 

4.17 

22° 

m4 

In  plotting  from  these  tables  a  vertical  scale  of  one-lialf  inch  per  one  per 
cent,  of  sulphur  oxidized  and  a  horizontal  scale  of  three-quarters  inch 
per  hour  of  oxidation  have  been  employed.  Wherever  there  was  a  drop 
in  the  results  the  gap  has  been  bridged  over  by  the  main  curve,  thus  mak¬ 
ing  the  curves  as  nearly  as  possible  progressive,  but  the  drop  has  been 
plotted  in  fine  dotted  lines.  On  examining  these  curves  the  points  as  to 
the  relative  rates  of  oxidation  that  have  been  mentioned  are  shown  very 
graphically,  the  curve  of  the  marcasite  oxidation  rising  in  each  case  above 
the  corresponding  one  of  pyrite,  but  both  showing  a  similar  form.  Of 
these  curves  the  normal  solution  cold  and  the  5  per  cent,  solution  hot 

*  Obtained  by  omitting  discordant  results. 


Plate  II. 


,il  aisil 


•-  QrravAe'  cpr/T'/rvo  <M^r*'r/Y!?' 

*  crjLCorarf^' 

- 


13 


are  the  most  regular.  No  very  marked  difference  in  the  rate  of  oxidation 
is  brought  out  by  this  series  of  experiments,  the  amount  of  sulphur  oxi¬ 
dized  never  having  reached  the  critical  point  in  pyrite,  as  shown  by  Prof. 
Smith’s  oxidations  with  the  current  already  described.  This  point  at 
which  the  rate  of  oxidation  of  sulphur  in  pyrite  suffers  a  change  was 
found  by  Prof.  Smith  to  be  between  21  and  22  per  cent,  from  the  results 
of  a  very  large  series  of  current  oxidations.*  The  explanation  for  this 
being  the  critical  point  in  the  oxidation  of  pyrite  will  be  given  in  the  dis¬ 
cussion  of  its  constitution.  The  experiments  with  permanganate  oxida¬ 
tion  simply  show  then  that  up  to  near  this  point  (the  highest  point  reached 
in  the  pyrite  oxidation  was  nearly  15  per  cent.)  the  relative  rates  of  oxi¬ 
dation  of  the  two  minerals  do  not  differ  widely  from  each  other,  but  that 
marcasite  oxidizes  somewhat  faster  than  pyrite.  This  is  simply  what  has 
been  long  known  and  recognized  in  regard  to  atmospheric  weathering. 

As  will  be  seen  when  the  constitution  of  these  minerals  is  considered, 
marcasite  cannot  have  a  critical  point  in  regard  to  oxidation  of  its  sulphur. 

The  experiments  thus  far  described  had  for  their  object  the  removal  of 
sulphur.  On  the  other  hand,  a  number  of  ways  of  attacking  the  iron 
were  tried  and  with  more  interesting  results.  In  these  trials  reagents 
were  selected  which  would  attack  the  iron  more  energetically  than  the 
sulphur.  Among  these  may  be  classed  the  experiments  of  solubility  in 
acids. 

Hydrochloric  acid  (hot  or  cold,  concentrated  or  dilute)  has  little  action 
on  these  minerals.  Pyrite  was  treated  for  one  hour  with  boiling  concen¬ 
trated  HC1,  of  specific  gravity  1.20  in  covered  beakers,  and  showed  in  the 
solution  only  2.56  per  cent,  of  iron  out  of  46.67  per  cent.  Marcasite, 
treated  in  the  same  way,  gave  an  identical  result.  Similar  experiments 
at  the  ordinary  temperature  were  tried  with  both  minerals,  by  digesting 
for  three  days  with  excess  of  concentrated  hydrochloric  acid  and  with 
excess  of  2  HC1  -f-  3H20,  but  even  after  three  days  the  action  was  very 
slight  in  both  cases.  Pyrite  gave  with  both  concentrated  and  dilute  acid 
the  same  result — a  solution  of  1.51  per  cent,  of  iron.  Marcasite  gave 
almost  identical  results.  The  concentrated  hydrochloric  acid  solution 
showed  1.51  per  cent,  of  iron,  the  dilute  solution  1.89  per  cent.  No  evo¬ 
lution  of  hydrogen  sulphide  was  detected  by  lead  paper  in  either  case. 
Concentrated  sulphuric  acid  at  boiling  temperature  decomposes  both 
minerals,  with  evolution  of  sulphur  dioxide  and  the  separation  of  sulphur, 
but  the  action  is  very  slow  and  seems  to  take  place  more  readily  with 
pyrite  than  with  marcasite.  Pyrite  digested  with  concentrated  sulphuric 
acid  at  boiling  temperature  for  one  hour  showed  14.81  per  cent,  of  the 
iron  dissolved,  but  marcasite  under  like  conditions  was  only  attacked  in 
one  hour  to  the  extent  of  12.77  per  cent,  of  iron.  Trials  were  also  made  in 
the  cold,  but  did  not  ditler  materially  from  the  results  obtained  with  HC1. 

More  important  results  were  obtained  by  conducting  dried  hydrochloric 

*  Private  communication  from  Prof.  E.  F.  Smith,  1893. 


14 


acid  gas  over  the  minerals  at  an  elevated  temperature.  In  these  trials, 
0.2  gram  of  the  mineral  was  placed  in  a  porcelain  boat  and  heated  in  a 
glass  tube  in  a  strong  stream  of  the  gas.  The  sulphur  in  the  series  of 
experiments  at  the  lower  temperature  was  collected  by  passing  the  gas 
through  bulbs  containing  Br  -f-  HC1 ;  at  the  higher  temperatures,  the 
residue  in  the  boat  was  analyzed  and  the  sulphur  lost  estimated  by  differ¬ 
ence.  In  the  experiments  at  low  temperature  the  entire  tube  was  exposed 
to  a  temperature  of  210°,  as  determined  by  thermometer.  The  HC1  was 
passed  over  in  a  strong  stream  for  one  hour.  The  action  at  this  tempera¬ 
ture  was  slight ;  the  results  obtained  do  not,  however,  show  the  entire 
amount  of  sulphur  removed,  as  some  remained  in  the  cool  end  of  the 
tube,  from  the  dissociation  of  the. hydrogen  sulphide.  As  the  action  was 
so  slight,  no  attempt  was  made  to  collect  and  estimate  this  sulphur 
remaining  in  the  tube.  In  the  bromine  and  hydrochloric  acid  solution 
was  found  sulphur  as  follows  : 


Pyrite  at  210°  in  current  of  HC1  (<z) .  0.94 

“  “  “  “  (6) . 0.93 

Marcasite  at  210°  in  current  of  HC1  («) . 0.77 

“  “  “  “  (5) .  0.59 


More  marked  results  were  obtained  by  increasing  the  temperature. 
Similarly  conducted  experiments  were  carried  out  at  310°  and  325°,  the 
time  of  heating  ranging  from  one  to  three  and  one-half  hours.  The  tem¬ 
perature  of  310°  was  graded  by  keeping  it  between  the  melting  points  of 
NaHS04,H20  (300°)  and  NaN03  (313°),  the  higher  temperature  was 
between  the  last  313°  and  the  melting  point  of  KC103  (334°).  After  the 
HC1  had  been  passed  for  a  sufficient  length  of  time,  the  tube  was  allowed 
to  cool  (with  the  gas  current  continued  until  cold)  and  then  the  remain¬ 
ing  sulphur  estimated  by  oxidizing  the  contents  of  the  boat  with  nitric 
acid  and  potassium  chlorate  and  precipitating  and  weighing  as  BaS04. 
The  amount  found,  subtracted  from  53.33  %,  gave  the  loss  of  sulphur. 
In  this  case  the  results  obtained  by  oxidation  were  reversed,  the  pyrite 
lost  more  sulphur  than  the  marcasite.  This  is  an  expression  of  the  fact  that 
the  hydrochloric  acid  gas  (or  its  contained  Cl)  acts  more  vigorously  on 
the  iron  of  pyrite  than  on  that  of  marcasite.  The  results  of  the  reaction 
were  in  each  case  ferrous  chloride  in  the  boat  and  free  sulphur  in  the 
tube,  the  latter  from  dissociation  of  the  hydrogen  sulphide.  No  ferric 
chloride  was  seen  in  the  tube,  except  a  trace  with  the  pyrite.  Each  min¬ 
eral  was  heated  for  one  hour  at  810°  in  a  current  of  the  gas  and  showed 
loss  of  sulphur  as  follows  : 

Pyrite  heated  at  310°  for  1  hour  in  HC1,  sulphur  lost . 10.73 

Marcasite  “  “  “  “  “  .  7.19 

About  the  same  relative  amounts  were  lost  on  heating  for  three  and 
one  half  hours  at  325°.  The  results  thus  obtained  were  as  follows  : 

Pyrite  heated  at  325°  for  3^  hours  in  IIC1,  sulphur  lost....  17.13 
Marcasite  “  “  ,f  “  “  10.70 


15 


Besides  these  two  experiments,  pyrite  was  heated  for  one  hour  at  a  red 
heat  in  a  stream  of  the  gas.  A  copious  sublimate  of  ferrous  chloride  was 
found  in  the  tube,  with  a  trace  of  ferric  chloride  and  sulphur.  This  time 
the  loss  was  46.47  per  cent,  of  sulphur.  It  seems  evident  from  these 
experiments  that,  as  above  stated,  the  iron  in  pyrite  is  in  a  condition  that  is 
more  readily  acted  on  by  hydrochloric  acid  than  is  the  iron  in  marcasite. 
It  will  be  proved  that  the  iron  in  marcasite  is  all  ferrous,  while  part  of  that 
in  pyrite  is  ferric,  and  this  is  probably  the  explanation  of  the  above  phe¬ 
nomenon.  All  of  the  iron  in  each  case  described  above  would  form  fer¬ 
rous  chloride  (FeCl2)  on  account  of  the  reducing  action  of  the  hydrogen 
sulphide  formed.  Under  the  conditions  of  the  above  experiments,  the 
critical  point  developed  in  the  oxidation  of  pyrite  was  not  reached,  but  it 
is  not  likely  that  it  exists  with  this  reagent,  or  if  there  be  a  critical  point 
it  is  not  21  per  cent.  The  thought  suggested  itself  to  me  that  perhaps 
some  sulphur  would  be  lost  in  pyrite  if  it  were  heated  to  325°  in  a  neutral 
atmosphere,  and  that  this  might  account  for  the  difference  shown  in  the 
loss  of  sulphur  in  the  two  minerals.  This  proved  not  to  be  the  case. 
Pyrite  heated  in  this  way  in  an  atmosphere  of  nitrogen  gave  no  appre¬ 
ciable  loss  after  one  hour  at  a  temperature  of  325°. 

Instead  of  hydrochloric  acid  gas,  the  action  was  tried  of  ammonium 
chloride  at  temperatures  up  to  335°  and  in  an  atmosphere  of  nitrogen. 
Under  these  conditions  the  sulphur  was  combined  as  ammonium  sulphide 
probably  and  did  not  exert  such  a  reducing  action  on  the  iron.  These 
experiments  were  conducted  as  follows  :  0.2  gram  of  the  finely  pulverized 
mineral  was  mixed  with  0.5  gram  dry  ammonium  chloride  and  introduced 
(in  a  porcelain  boat)  into  a  glass  tube.  Test  samples  of  NaHS04'  H20 
and  KC103  in  sealed  tubes  were  used  to  regulate  the  temperature.  All 
air  was  displaced  in  the  tube  by  nitrogen  and  a  slow  current  of  nitrogen 
passed  through  the  tube  before  heating.  Under  these  conditions  with 
marcasite,  sulphur  and  ammonium  sulphide  were  found  sublimed  in  the 
tube  along  with  ammonium  chloride,  and  in  the  boat  there  was  found 
much  ferrous  chloride  without  any  ferric  chloride,  but  in  the  case  of  the 
pyrite  there  was  formed  a  large  proportion  of  ferric  chloride,  which  sub¬ 
limed  on  the  tube  towards  the  end  of  the  operation.  The  heating  was 
conducted  slowly  in  each  case  and  continued  until  all  ammonium  chloride 
was  sublimed  from  the  boat,  the  temperature  of  335°  not  being  exceeded 
during  this  time.  The  entire  operation  lasted  about  twenty-five  minutes 
in  each  case.  Three  trials  of  each  mineral  were  made  and  with  the  same 
result  in  each  case  ;  with  marcasite  only  ferrous  chloride  was  found  in  the 
boat  and  no  iron  in  the  tube,  pyrite  always  gave  much  ferric  chloride  and 
little  ferrous.  The  amounts  of  sulphur  removed  are  probably  not  very 
significant ;  they  showed  the  following  results  : 


Pyrite  heated  with  NII4C1  lost  sulphur  ( a ) . 7.02 

“  “  “  “  (6) . 7.10 

Marcasite  “  “  “  (a) . 9.50 


16 


The  important  point  brought  out  in  these  experiments  is  that  pyrite 
contains  a  large  amount  of  ferric  iron,  while  in  marcasite  the  iron  appar¬ 
ently  exists  in  the  ferrous  condition.  Some  reducing  action  might,  how¬ 
ever,  have  taken  place,  due  to  the  sulphides  formed.  The  condition  of 
the  iron  in  the  chlorides  found  in  the  boat  and  tube  was  very  carefully 
tested  by  several  reagents  in  each  case,  and  there  can  be  no  doubt  as  to 
the  correctness  of  the  results  as  stated  above. 

The  decomposition  of  the  sulphides  by  metallic  salts  seemed  to  offer 
some  hope  of  being  productive  of  results  that  would  show  in  a  quantita¬ 
tive  way  the  exact  amounts  of  ferrous  or  ferric  iron  that  are  present  in 
these  two  minerals.  In  this  line,  the  action  of  gold  chloride,  silver 
nitrate  and  silver  sulphate  were  tried  in  a  qualitative  way  with  both,  min¬ 
erals.  Of  these,  the  first  gave  a  ready  decomposition  with  both,  and 
produced  both  ferrous  and  ferric  salts  in  each  case.  The  silver  nitrate 
gave  a  similar  result.  Silver  sulphate  acted  very  slowly  and  without  any 
definite  results. 

The  action  of  copper  sulphate  in  neutral  solution  and  under  pressure 
was  tried  with  very  remarkable  results.  At  the  ordinary  temperature 
and  pressure  the  solution  of  this  salt  has  little  effect  on  either  mineral, 
and  the  same  is  true  of  the  solution  at  a  boiling  temperature,  but  under 
pressure  the  reaction  is  complete.  The  experiment  was  conducted  as 
follows  :  0.2  gram  of  the  finely  pulverized  mineral  was  introduced  into  a 
stout  glass  tube,  and  50  c.c.  of  a  10  per  cent,  solution  of  the  salt,  CuS04‘- 
5EI20,  added,  the  air  displaced  with  a  pinch  of  NaC03  and  a  drop  or  two 
of  H2S04  (dilute),  and  a  heavy  seal  made  on  the  tube.  The  tubes  con¬ 
taining  the  two  minerals  were  heated  for  six  hours  in  an  autoclave 
to  a  temperature  of  about  200°.  The  contents  of  the  tubes  were  found 
to  contain  no  traces  of  undecomposed  mineral,  but  there  was  a  black, 
more  or  less  flocculent  precipitate  in  its  place.  This  proved  to  be  copper 
sulphide.  The  solution  had  not  altered  appreciably  in  appearance.  The 
liquid  contents  of  the  tube  wTere  in  each  case  transferred  to  a  flask  pre¬ 
viously  filled  with  C02  and  with  10  c.c.  dilute  sulphuric  acid  in  the  bot¬ 
tom,  the  tube  then  rinsed  with  water  and  the  amount  of  ferrous  iron 
present  titrated  with  freshly  standardized  potassium  permanganate.  In 
the  case  of  marcasite  this  gave  18  c.c.  KMn04  solution  (this  was  two-  or 
three-tenths  of  a  cubic  centimeter  too  much,  on  account  of  the  difficulty 
in  catching  the  end  reaction).  To  correct  this  for  the  iron  in  the  copper 
sulphate,  a  blank  of  50  c.c.  CuS04  solution,  the  same  as  used  above  with 
10  c.c.  dilute  sulphuric  acid,  was  titrated  with  the  permanganate,  giving 
0.5  c.c.  reduction.  The  factor  of  the  permanganate  was  .0054  gm.  Fe  for 
1  c.c.  Making  the  correction  for  the  reduction  of  50  c.c.  CuS04  solution, 
this  gives  47.25  per  cent,  of  iron  in  solution  as  against  46.67,  the  theoreti¬ 
cal  amount  in  FeS2.  No  doubt  if  the  end  reaction  had  been  more  exact 
there  would  have  been  a  still  closer  correspondence  in  the  result. 

The  tube  containing  the  pyrite  was  treated  in  exactly  the  same  manner, 
and  gave  a  reduction  of  permanganate  of  3.8  c.c.  This  time  the  end 


17 


reaction  was  sharp  and  exact.  Calculating  the  above  to  iron  (after  mak¬ 
ing  correction  for  CuS04)  this  gives  8.91  per  cent,  of  ferrous  iron  in 
pyrite.  As  the  total  iron  is  4G.67,  this  corresponds  to  19.09  per  cent,  of 
the  iron  in  the  mineral,  or  almost  exactly  one-fifth.  These  experiments 
demonstrate  in  a  positive  manner  the  condition  of  the  iron  in  the  two 
minerals,  and  even  show  the  exact  amounts  of  each  condition  of  the 
iron,  ferrous  and  ferric.* 

That  marcasite  should  hence  be  more  readily  decomposed  by  oxidation 
than  pyrite  seems  fully  explained  by  the  foregoing  investigations,  as  it 
consists  of  Fe/7S2,  an  unsaturated  compound.  In  this  compound,  sulphur 
must  link  to  sulphur,  or  the  compound  have  unsaturated  bonds,  and 
hence  any  element  which  would  attack  the  sulphur  would  break  up  the 
compound.  On  the  other  hand,  the  iron  is  held  to  the  sulphur  by  its  full 
number  of  bonds,  and  any  substance  that  has  an  affinity  for  iron  could 
not  so  readily  attack  it  in  this  condition.  This  would  be  true  whether 
ferrous  iron  be  considered  here  as  Fe2,  with  a  valence  of  four,  or  as  Fe". 
That  marcasite  is  Fe'^  is  also  indicated  by  its  oxidation  in  the  air  into 
FeS04  mainly.  Under  these  same  conditions  it  will  be  noted  that  pyrite 
forms  both  ferrous  and  ferric  compounds,  as  FeS04,  but  much  more 
Fe403  (OH)6  and  free  sulphur.  Marcasite,  however,  when  decomposed 
by  water  under  pressure  (in  nature)  forms  much  limonite  also,  this  being 
due  no  doubt  to  the  oxidation  being  effected  under  pressure.  This  con¬ 
stitution  explains  also  the  fact  that  the  oxidation  of  marcasite  is  continu¬ 
ous  and  complete,  as  shown  by  the  current  oxidations.  It  will  be  shown 
also  that  this  constitution  of  pyrite  that  has  been  made  out  explains  fully 
its  action  with  the  current.  That  marcasite  is  unsaturated  is  also  indi¬ 
cated  by  the  fact  that  it  has  not  been  made  artificially  or  at  any  rate  posi¬ 
tively  identified  in  any  of  the  artificial  FeS2  that  has  thus  far  been  made. 
If  marcasite  be  a  persulphide,  as  its  formula  would  seem  to  indicate  for  a 
ferrous  compound,  none  of  the  methods  detailed  above  for  making  FeS2 
would  be  applicable  in  its  case,  unless  perhaps  the  method  of  Deville 
might  produce  it.  All  of  the  other  methods  would  probably  produce 
ferric  iron,  at  least  in  large  part,  and  the  resulting  product  would  be 
pyrite. 

The  formula  for  pyrite  derived  from  my  investigations  and  expressing 
the  relation  of  the  two  conditions  of  the  iron  in  the  simplest  way  is 
4  FeivS2-  Fe"S2.  This  formula  is  also  borne  out  by  what  we  know  of  the 
formation  of  pyrite  as  given  in  the  early  part  of  this  paper,  and  by  such 
experiments  as  I  have  made  on  its  decomposition,  as  well  as  the  fact  above 
alluded  to  that  it,  in  oxidizing  in  nature,  does  not  form  much  ferrous 
compounds  but  mainly  ferric.  And  it  also  explains  the  fact  that  it  is  more 
stable  as  regards  any  element  attacking  its  sulphur,  for  it  is  most  probable 
that  all  the  sulphur  of  the  Fe'^  in  its  formula  is  linked  to  iron.  I  would 
propose  the  following  structural  formula,  not  as  expressing  the  exact  con¬ 
stitution  of  the  compound,  for  of  that  we  know  nothing,  but  as  an 

*  These  results  have  been  confirmed  by  experiments  made  during  the  past  year  in  this 
laboratory,  and  not  yet  published. 


18 


UNIVERSITY  OF  ILLINOIS-URBANA 


372363044 


expression  of  the  condition  of  the  iron  in  the  molecule  and  as  embodying 
in  a  quantitative  way  the  result  of  my  investigations  into  its  constitution. 
It  will  be  noticed  that  the  sulphur  of  the  Fe//S2  is  made  to  link  entirely 
with  iron. 

S 

Fe< 

S 

I 

8 

8— Fe< 

X  S 

Structural  formula  of  pyrite.  Fe" 

S 

8— Fe< 

S 

I 

s 

Fe^ 


If  ferric  iron  be  considered  as  Feiv — Feiv  it  is  only  necessary  to  connect 
the  ferric  Fe  atoms  with  bonds,  but  it  seems  to  me  that  ferric  iron  is  more 
likely  Fe///,  and  at  any  rate  this  is  the  simplest  way  to  regard  it.  A  very 
striking  proof  of  the  correctness  of  the  idea  expressed  in  this  structural 
formula  that  Fe"S2  in  pyrite  has  its  sulphur  all  linked  to  iron  is  afforded 
in  the  experiments  on  oxidation  of  the  mineral  by  means  of  the  electric 
current  as  detailed  above.  It  will  be  recalled  that  the  amount  thus 
oxidized  was  between  21  and  22  per  cent.  Now  if  two  molecules  of  FeS2 
be  split  off  from  the  above  formula,  say  those  linked  by  sulphur  to  sulphur 
there  would  remain  a  saturated  compound  much  more  difficult  to  decom¬ 
pose  (theoretically)  than  the  pyrite  molecule  illustrated  and  the  amount 
of  sulphur  thus  removed  would  be  by  calculation  21.33  per  cent.  This 
action  could  be  thus  illustrated  : 


Fe- 


Fe' 


S— Fe< 

S 

decomposed 

,  Fe< 
S7 

1 

by  the  current 

Fe< 

8 

S— Fe< 

would  give 

S\ 

XFe< 

Fe« 


Of  course  this  structural  formula  is  only  intended  to  represent  the 


